1. Field of the Invention
The present invention relates to a layered bandpass filter including three or more resonators that are provided within a layered structure including a plurality of stacked dielectric layers.
2. Description of the Related Art
Recently employed communication devices include those for wireless LANs (local area networks), those compatible with Bluetooth® technology, those compatible with WiMAX® (worldwide interoperability for microwave access) technology, and cell phones. While required to be reduced in size and thickness, these wireless communication devices are each required to process signals of systems that are in various different frequency bands. Electronic components used in these wireless communication devices include a bandpass filter which filters transmission and reception signals. Band-pass filters are also required to be reduced in size and thickness. A bandpass filter that is responsive to usable frequency bands of the aforementioned wireless communication devices and can be reduced in size and thickness is disclosed, for example, in each of JP 2006-033614A, JP 2006-054508A, and JP 11-041004A. The bandpass filter is a layered bandpass filter with a plurality of resonators provided within a layered structure including a plurality of stacked dielectric layers. In the layered bandpass filter, two adjacent resonators are electromagnetically coupled to each other. The electromagnetic coupling includes inductive coupling and capacitive coupling.
The layered bandpass filter disclosed in each of JP 2006-033614A and JP 2006-054508A includes three or more resonators, two adjacent ones of which are electromagnetically coupled to each other. In the layered bandpass filter, a coupling path including a capacitor is provided to form capacitive coupling between two resonators not adjacent to each other. In terms of pass attenuation characteristics of a bandpass filter, the provision of such a coupling path allows the formation of an attenuation pole at a frequency lower than the passband frequencies. This provides better attenuation characteristics of the bandpass filter at frequencies lower than those of the passband.
As disclosed in JP 2006-033614A and JP 2006-054508A, if the layered bandpass filter including three or more resonators is provided with the coupling path for forming capacitive coupling between two resonators not adjacent to each other, spurious may occur at a frequency higher than the passband frequencies due to the provision of the coupling path. A possible reason for the occurrence of spurious is as follows. The aforementioned coupling path includes, in addition to the capacitor, a conductive path that is connected in series to the capacitor. The conductive path has inductance, and is therefore considered as an inductor. Accordingly, the coupling path is considered as a series resonant circuit formed by the capacitor and the inductor. The resonant frequency of the series resonant circuit is generally higher than the passband frequencies. As a result, in terms of pass attenuation characteristics of the bandpass filter, a peak of reduction of attenuation (hereinafter referred to as low attenuation's peak) occurs at a frequency higher than the passband frequencies due to the presence of the series resonant circuit. The frequency at which the low attenuation's peak occurs is the resonant frequency of the series resonant circuit. The low attenuation's peak degrades the attenuation characteristics of the bandpass filter at frequencies higher than those of the passband. The low attenuation's peak especially generates spurious of a frequency that is the frequency of the low attenuation's peak, or a frequency close to the frequency of the low attenuation's peak.
The low attenuation's peak occurs noticeably if the layered structure is composed of a plurality of dielectric layers of a high permittivity in order to respond to a need to reduce insertion loss in the passband, in particular.
A bandpass filter used in wireless communication devices is required to reduce insertion loss in the passband, and in some cases, is required to have such a characteristic that attenuation in a specific frequency band within the stopband is equal to or higher than a predetermined level. For example, in the case of a wireless communication device for processing signals of a plurality of systems in various different frequency bands, a bandpass filter used in one of the systems may be required to have such a characteristic that attenuation in a frequency band of a signal of a different one of the systems is equal to or higher than a predetermined level. In this situation, if the frequency at which the aforementioned low attenuation's peak occurs is within the frequency band in which attenuation is required to be equal to or higher than a predetermined level, the bandpass filter may fail to achieve its desirable characteristics.
This may be prevented by reducing at least either the capacitance of the capacitor or the inductance of the inductor in the series resonant circuit to shift the resonant frequency of the series resonant circuit, namely, the frequency at which the low attenuation's peak occurs, to a higher side than the frequency band in which attenuation is required to be equal to or higher than a predetermined level. However, reduction in capacitance or inductance may result in a failure to form an attenuation pole at a frequency lower than the passband frequencies, or may change the frequency or attenuation of the attenuation pole. In either case, the coupling path fails to fulfill its original function.
The bandpass filter disclosed in each of JP 2006-033614A and JP 2006-054508A includes two adjusting electrodes for forming capacitors between the adjusting electrodes and two resonant electrodes not adjacent to each other, and a connecting electrode for electrically connecting the two adjusting electrodes to each other. Each of these publications also discloses the technique of reducing spurious (low attenuation's peak) occurring at a frequency higher than the passband frequencies by reducing electrostatic capacitance generated between the connecting electrode and a resonant electrode that is provided between the two resonant electrodes not adjacent to each other. However, this technique cannot shift the frequency at which the low attenuation's peak occurs to the outside of the frequency band in which attenuation is required to be equal to or higher than a predetermined level.
JP 11-041004A discloses the technique of shifting the position of occurrence of undesired resonance, which occurs at a frequency higher than the passband frequencies due to the presence of a conductor layer for forming capacitive coupling between adjacent resonators, to a still higher frequency. However, this publication does not disclose a coupling path for forming capacitive coupling between two resonators not adjacent to each other, nor any solution to problems relating to the low attenuation's peak generated by the provision of the coupling path.